Abstract: A diathermy apparatus includes a cylindrical coil (24) defining an opening (42), a pair of capacitors of substantially similar capacitance (20 and 22), each capacitor connected to one end of the coil, an RF signal source (26), and a balun (28) connected between the RF signal source (26) and the pair of capacitors (20 and 22). The balun (28) is operative for converting an unbalanced signal received from the RF signal source (26) into a balanced signal supplied to coil (24) via the pair of capacitors (20 and 22). Desirably, the cylindrical coil is a conical cylindrical (or cone-shaped) spiral coil.